As part of the Session Initiation Protocol (SIP) message flow to set up a call (or ‘communication session’), an end-point can optionally include SDP data to suggest the media to use (for example choice of codec). That SDP data can also contain one or more “preconditions” on the media (for example reservation of resources and/or securing a minimum Quality of Service (QoS)).
The Voice over Long Term Evolution (VoLTE) specifications mandate that phones use SIP preconditions, and that legacy equipment should be backwards compatible with receiving SIP preconditions. However, some legacy equipment is not backwards compatible. To ensure that calls do not fail, 3rd Generation Partnership Project (3GPP) specifications therefore allow networks to mandate either that all incoming SIP INVITE messages must contain preconditions, and reject any that do not with a SIP error code indicating such, or that all incoming SIP INVITE messages must not contain pre-conditions, and reject any that do with a SIP error code indicating such.
A network operator can choose which behavior they want to match the behavior of their back-end equipment.
For any given SIP INVITE message, intermediate network elements may or may not receive preconditions, and may or may not need to send preconditions. Hardcoded configuration, for example always send preconditions or always do not send preconditions, will only work in some scenarios, for example where all downstream endpoints on a given adjacency have the same behavior, and not the general case.